I came away from this year's Turner prize show thinking, "This won't do, will it?" It was more than the downbeat and understated air of the event. For those of us familiar with the four artists and their recent exhibitions, too much of the work has been seen before. Two of Richard Billingham's videos, Ray in Bed (1999) and Tony Smoking Backwards (1998), as well as a photographic triptych of Ray's hands, were in the artist's show at the Ikon Gallery in Birmingham last year; the show finished an international tour this June. Both of Isaac Julien's films, The Long Road to Mazatlan and Vagabondia, have toured over the past 12 months. Martin Creed's single work at Tate Britain, Work Number 227: The Lights Going On and Off (it is exactly as the title describes it), is dated 2000, but is actually no differ ent in concept from a work of the same title, designated Work Number 127, that was first presented at the Cubitt Gallery in London in 1995.

Or is it different? Up in the middle of each truncated pyramid skylight are fluorescent strips hidden behind translucent sheets. Around the edge of these cowls are lighting tracks holding six spotlights per unit. The spotlights come on - suddenly. The hidden fluorescents power up a nanosecond later. It's bright for a bit, then the lights go off in a fast, even fade - an effect achieved through pure electromechanics rather than the artist's tinkering. You think about the semi-darkness for a bit, then the lights come on again.

Creed's work is different every time it is shown, in each new venue, each with its own lighting fixtures and conditions. But do we care? It gives a critic sleepless nights. Martin Creed's open, empty double space is not so much a surprise in itself - the empty or near-empty gallery has a long, if not exactly venerable history - and having the lights go on and off has a certain esprit, or at least it did the first time. Now it feels like a singularly ungenerous use of an opportunity.

Maybe Creed believes that this is a rigorous and brave thing to have done: getting back to a kind of stripped-down statement after entertaining us with too many of his recent and rightly popular pieces. You could say that this work has something to say about the visibility of the artwork, about presence and absence (and about something and nothing) - that there is no light without shadows. But I would much rather you didn't say anything remotely like that.

Julien's two films are shown in adjacent rooms. One is painted a deep, chemical red, the other white. Each space is half-filled with screens and projection equipment, housed in units that have a sci-fi, baroque, minimalist presence. Do Julien's two works belong together in this way? What is the relationship between the manner of presentation and the work's content? Dramatic though these screening chambers are, I can't help but think that they actually add little, except as a framing device.

Perhaps one of the problems the artists had this year was obtaining funding for new work. But, that said, Julien's film and video works take years to complete. Creed is a slow worker, even if his ideas in themselves appear to be the work of a moment.

Julien's Vagabondia and The Long Road to Mazatlan both feel different at the Tate. It is mostly a matter of being closer to the screen, but there are technical improvements as well: sharper projection, crisper and more resonant sound quality than before. Both films are immensely seductive works, enriched by multiple viewings. However, there are tiresome longueurs in Mazatlan - if I see those leg-kicking, pouting, camera-conscious hitch-hiking saloon girls (choreographed by Javier de Frutos) one more time, I shall scream.

One of the things Julien has always been effortlessly good at is filmic ravishment, and there is a moment in Vagabondia, set in London's Sir John Soane's Museum, when the heavy pleats of Cleo Sylvestre's long red dress fill the split screen - in a red frame, in a red room. In this moment, everything comes together magically. You are sucked into the fold of the mirror reversals of the image. It is as like a painting as video can get.

Mike Nelson's installation, portentously titled The Cosmic Legend of the Uroboros Serpent, is part of a shifting terrain in which objects, spaces and places are constantly recycled. He recycles authors too - Borges, Stanislaw Lem, HP Lovecraft, William Burroughs - along with a growing collection of detritus, junk, old doors and little piles of thrown-out objects.

It is as if every work Nelson has made is part of the same piece. Every corridor and corner he constructs lead, somehow, to every other place he has built or installed elsewhere. It is not even a sealed world of his own invention: one can imagine that his installations lead by some circuitous route into Ilya Kabakov's installations, into Ed Kienholz's seedy bars and mental asylums, into a Paul McCarthy or Mike Kelley set. Having recently seen Nelson's labyrinths of interconnecting, squalid rooms at Matt's Gallery in London, in Venice and at London's ICA, I find that excitement dulls into boredom.

The best thing about Nelson's Tate installation - as in his other works - is its verisimilitude. You forget where you are, becoming disoriented as you emerge through one last door and imagine yourself back where you began. It's an illusion; retracing your steps, you arrive at a dead end, and a mop in a dirty, galvanised bucket.

Followers of Richard Billingham's work might both expect and want more of the work that has deservedly made his name: more photos of fat Liz and drunk Ray, more Jason swatting flies, and the cat - not levitating, but hurled across the room. Ray is here, twice: first in bed for an afternoon nap, and then in a large triple image, with beautiful focused and unfocused details of his hands. They are like a landscape.

Both video and photographs home in on surfaces: Ray's pores, knotted veins, thin hair; jumpers piled in the corner; floral wallpaper; Liz's flowery dress. Ray in Bed is the afternoon torpor of a middle-aged man. Tony Smoking Backwards, with a drizzle of ambient background music, dwells on the thick smoke going in and out, the young smoker's mannerisms, his indolence.

The surprise is that Billingham does not give us entirely what we might expect. There are three new photographs here: a hillside and distant village in Cephalonia; thick foliage and trees overhanging what seems to be an overgrown and silted-up ornamental lake; a girl on a beach in bright sun (pictured, above). Around the lounging girl, arms outstretched, are the photographer's footprints, where he has stalked her pose for the camera.

It is difficult not to read these images as signs of Billingham's escape from an overwhelming subject. They are almost idyllic. But it is also difficult not to imagine that, back in that village, there's a family like the Billinghams, having it out behind closed doors. His show seems to me a halfway station between two worlds, but there is at least a sense of something being made new again.

For me, the prize is between Billingham and Julien. Julien has struggled for years with the conflicts between making art and wanting to make movies, between theories and pleasure in the visual. Billingham is perhaps similarly conflicted, but about future content rather than the form of his work. With a little regret, I believe he should win.